As known, the containers of the above mentioned type, after having been filled with hot—for example at about 85° C.—pourable products or liquids, are first subjected to a capping operation and then cooled so as to return to a room temperature. By effect of the capping operation, the heated air present in the top portion (“head space”) of the container expands causing a stress tending to produce a general swelling of the container at the side wall and at the base wall.
The following cooling to which the container is subjected, causes, vice versa, a reduction of the volume of air and minimally of the liquid product contained in the container; a depression is therefore created, which tends to pull the side walls and the base wall of the container inwards. This may determine deformations in the walls of the container if these are not rigid enough to resist the action of the above disclosed stresses.
In order to contain the depressive stresses generated during the cooling of the product within the containers without generating undesired deformations on the containers, they are typically provided, at the side wall, with a series of vertical panels, known as “vacuum panels”. These panels, in the presence of depressive stresses, are deformed inwardly of the container allowing it to resist to the hot fill process without generating undesired deformations in other areas of the container.
Likewise, the known containers intended to be subjected to a hot fill process can also have an optimised lower portion or base adapted to be deformed upwards under the action of the depressive stresses.
Even though the disclosed solutions allow to “relieve” the pressure stresses on specific parts of the containers, i.e. the vertical vacuum panels or the base, thus avoiding the occurrence of undesired deformations in other parts of the containers, they do not allow the cancellation of the above said stresses; in other words, the containers remain in any case subject to internal depressive stresses and must therefore be provided with a structure capable of resisting such stresses.
Patent application WO2006/068511 shows a container having a deformable base, which can have two different configurations: a first unstable configuration, in which this base has a central area projecting downwards with respect to the outermost annular peripheral area immediately adjacent thereto, and a second stable configuration, in which the central area is retracted inwardly of the container, i.e. it is arranged in a higher position with respect to the adjacent peripheral area.
Following the filling with the hot pourable product, the base of the container has the first unstable configuration and must be supported by a special cup element to which it is coupled. Thereby, the downward deformation of the base of the container can be maximised without compromising the stable support of the container, since such a support is provided by the cup element. Following the cooling, the base can be deformed by an external action, for example a vertical thrust upwards performed by a rod or plunger, in the second stable configuration with the subsequent possibility of removing the cup element.
The deformation of the base of the container from the first to the second configuration determines a considerable reduction of the containment volume of the container, much higher than would be obtained in the known containers simply by the deformation of the base by the effect of the sole depressive stresses; the final effect is therefore substantially the cancellation of the depressive stresses acting on the inside of the container.
The applicant has observed that this kind of operation is quite critical, as the final configuration of the container after the deformation depends on a plurality of factors, connected not only to the way in which such operation is performed but also to the initial condition of the container subjected to deformation.
In particular, the applicant has observed that a non-correctly filled container, i.e. a container overfilled or filled below the desired level, may react to the deforming action of the plunger in a non predictable way: for instance, an overfilled container may be deformed insufficiently at its base, with possible return of the plastic material towards the original first configuration after release of the plunger; alternatively, a non-sufficiently filled container may be broken by the action of the plunger.
It should be also noted that an incorrect deformation of the container may affect the subsequent application of the label. As known, a label requires a receiving surface having a well-defined geometry as well as a sufficient rigidity. This second feature of the receiving surface is particularly important for self-stick labels or pressure-sensitive labels.